Ionomers are polymers that contain a small concentration of covalently bonded ionic species, such as carboxylate, sulfonate or phosphonate groups. In most cases, “hard” counterions, such as metal ions, are used to form the ion-pair, which in ionomers is condensed because of the relatively low dielectric constant of the polymer matrix. This characteristic, in addition to the relatively low ion density, distinguishes ionomers from polyelectrolytes, which are highly charged polymers that are usually water soluble. The interest in ionomers stems from the large property changes that result from interchain supramolecular bonding of the contact ion-pairs. These interactions represent transient, reversible crosslinks that generally increase the modulus, strength, and toughness of the ionomer, though some extensibility of the parent polymer is lost due to the formation of a physical network. The presence of the ionic groups and phase-separated ionic nanodomains, often termed ionic clusters, that form in most ionomers also affect the glass transition temperature and the transport properties of the material.
Less common is the addition of ionic functionality to a polymer for the purpose of internally plasticizing the polymer. This can be achieved by using bulky counterions, e.g., alkyl ammonium or phosphonium ions that weaken the ionic, dipole-dipole, or ion-dipole interactions responsible for the mechanical and physical property changes. For example, Weiss et al. and Weiss and Stamato7 used alkyl ammonium cations with varying alkyl chain lengths to lower the glass transition and melt viscosity of sulfonated polystyrene ionomers.
Thermoplastic polyurethanes (TPU) are linear segmented block copolymers that possess polar hard segments derived from diisocyanates, such as methylene diphenyl diisocyanate (MDI), and relatively non-polar soft segments formed from oligomeric diols, such as polyesters and polycarbonates. The disparity in the polarity of these two segments and the crystallizability of the hard segment produces microphase separation of the hard segments into nanodomains that provide physical crosslinks responsible for the desirable properties of TPUs, such as excellent elasticity, abrasion resistance, and toughness. Crystallization of the hard segments also increases the modulus and hardness (durometer) of a TPU. TPUs with lower durometer are normally achieved by adding low molecular weight plasticizers, such as dipropylene glycol dibenzoate or benzoate esters. However, those compositions tend to be tacky and are often difficult to process for use in common TPU applications. In addition, plasticizer leaching and migration is a major industrial challenge that eventually leads to a decline in the thermal and mechanical properties of plastics, and has also brought about serious health and environmental concerns. As a result, increasing restrictions on the use of traditional plasticizers have created a demand for alternative methods for softening TPUs.
Accordingly, what is needed in the art is a plasticizer, and related method for softening TPUs for processing, that does not leach or migrate out of the TPU or bring about a decline in its thermal and mechanical properties.